LAUGHTON
ELECTRONICS

New Spin for a Leslie
modifying the input stage on a classic rotating speaker

Here
is an old vacuum-tube device I modified. The goal was to provide a
low-level unbalanced input and to find alternative means for dealing
with power and motor-control connections, which were originally
configured to connect to a Hammond organ.

The Leslie 145
is
an amplifier-speaker
combo with rotating baffles that give it the classic, tremolo electric
organ sound. But the owner of this particular unit wanted the freedom
to use it with electric guitar and other instruments (and of course
organ as well).

There were three main aspects to deal with. The most obvious matter was
to add a preamp stage that would allow the unit to
accept a low-level unbalanced input signal via a standard 1/4 inch
jack. This entailed adding an extra tube (a 12AX7 dual triode) to act
as an input buffer and phase inverter.

The second issue was dealing with power switches and power
connections. Something was needed to replace the original setup, in
which a switched AC source simply arrived via a cable and octal plug
from the
organ. (The same plug carried the balanced audio lines and the motor
speed select.)
My solution was to add a female EIA power connector so that a
standard, removable (and replaceable) power cable could be used;
this also provided a safety ground connection which the unit
otherwise lacked. I installed one toggle switch for Mains switching,
and added another to provide a Standby function as well.

The final issue was some means to provide remote switching for
the fast/slow control for the baffle motors. I decided to leave
the options open, since the actual switch could take many forms: a
footswitch, for instance, or a toggle located on the organ console. I
wired up a low-voltage, solid state circuit that accepted its input
through an XLR plug that mounts where the old octal socket used to be.
The idea of using XLR was that an ordinary mike cable could be used as
an extension for the footswitch circuit, although it's maybe a bit
unlikely that that'll ever happen. Anyway, there it is.